CONTRACT REPORT
Theoretical Evaluation of the NightCool
Nocturnal Radiation Cooling Concept
FSEC-CR-1502-05
April 2005
DOE Award No. DE-FC26-99GO10478
FSEC Contract No. 20126003
Submitted to:
U.S. Department of Energy
Submitted by:
Danny S. Parker
Theoretical Evaluation of the NightCool
Nocturnal Radiation Cooling Concept
FSEC-CR-1502-05
April 2005
Submitted to:
U.S. Department of Energy
Submitted by:
Danny S. Parker
Florida Solar Energy Center
1679 Clearlake Rd.
Cocoa, FL 32922
Table of Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Theoretical Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Empirical Data on Potential of Nocturnal Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Description of the NightCool Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Description of the Radiative Cooling Prediction Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Predicting Roof/Attic Radiator Heat Exchanger Performance . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Roof Convective Heat Transfer to Outdoors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Weather Data and Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Roof Radiator Surface Temperature Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Dewpoint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Emissivity and Spectral Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
View Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Cloudiness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Return Air Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Return Air Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Fan Motor Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Analysis of Cooling Potential Using Simulation Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Simulation in Other Climates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Evaluation in Small Scale Test Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Acknowledgment
This work is sponsored, in large part, by the US Department of Energy (DOE), Office of Energy
Efficiency and Renewable Energy, Building America Program under cooperative agreement number
DE-FC26-99GO10478. The authors appreciate the encouragement and support from Mr. George
James and Mr. Chris Early, DOE program managers in Washington, DC. and Mr. Bill Haslebacher,
DOE project officer at the DOE National Energy Technology Lab, Morgantown WV. At Florida
Solar Energy Center Rob Vieira and Subrato Chandra helped with report review. Raju Sen Sharma
and Lixing Gu assisted with key aspects of the computer simulation. John Sherwin assisted with the
experimental test plan.
Disclaimer
This report was prepared as an account of work sponsored by an agency of the United States
government. Neither the United States government, nor any agency thereof, nor any of their
employees, makes any warranty, express or implied, or assumes any legal liability or responsibility
for the accuracy completeness, or usefulness of any information, apparatus, product, or process
disclosed, or represents that its use would not infringe privately owned rights. Reference herein to
any specific commercial product, process, or service by trade name, trademark, manufacturer, or
otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring
by the United States government or any agency thereof. The views and opinions of authors
expressed herein do not necessarily state or reflect those of the United States government or any
agencies thereof.
heoretical Evaluation of the NightCool Nocturnal Radiation Cooling Concept
Danny S. Parker
April 2005
Abstract
An evaluation has been conducted of the potential of a night sky cooling system to substantially
reduce space cooling needs in homes in North American climates. The report describes the
innovative residential cooling system which uses nocturnal night sky radiation from a roof integrated
radiator. The system uses a sealed attic covered by a highly conductive metal roof which is
selectively linked by air flow to the main zone with the attic zone to provide cooling– largely during
nighttime hours. Available house mass is used to store sensible cooling. Additional dehumidification
is done during the evening hours as warranted by an interior control humidistat.
The report describes a detailed simulation model of the relevant physical night cooling phenomenon,
examining each particular parameter which was found to have an appreciable impact on
performance. A 225 square meter metal roof structure is modeled in Tampa, Florida. Under a series
of standard nighttime conditions approximating humid nighttime summer weather, the model
predicts a cooling rate of about 2,140 Watts (7,300 Btu/hr). The model features several
enhancements (such as constraining the radiator temperature to the dewpoint temperature) never
before incorporated into such a model. The report finds that the major weather-related influences
on achieved cooling performance are outdoor air temperature, dewpoint temperature, cloudiness and
wind speed. Physical factors with a large influence are the system return air temperature (and hence
radiator temperature) air flow rate and fan and motor efficiency. For Tampa, Florida, the model
predicts an average summer cooling benefit of about 15 kWh per day for 1.4 kWh of fan power for
a system seasonal energy efficiency ratio (SEER) of about 37 Btu/Wh. Performance in less humid
climates with more diurnal temperature swing was predicted to be substantially better. A follow-up
experimental plan is described to obtain empirical data on concept performance using two highly
instrumented test sheds.
1
Theoretical Evaluation of the NightCool Nocturnal Radiation Cooling Concept
Executive Summary
Using a building’s roof to take advantage of long-wave radiation to the night sky has been long
identified as a potentially productive means to reduce space cooling in buildings. The night cooling
resource is large and enticing for residential energy-efficiency applications. On a clear desert night,
a typical sky-facing surface at 80EF (27EC) will cool at a rate of about 75 W/m2. In a humid climate
with the greater atmospheric moisture, the rate drops to about 60 W/m2. Night-time cloud cover is
an important variable as well. With 50% cloud cover in a humid climate, the cooling rate drops to
about 40 W/m2 and only about 7 W/m2 under completely overcast skies. Average potential daily July
cooling for a radiator at 72EF amounts to 63 - 110 Wh/m2 of roof surface in U.S. climate locations
– the lower value being representative of a humid region like that in Florida (Clark, 1981).
For a typical roof (225 square meters), this represents a cooling potential of 6,000 - 14,000 Watts
or about 1.5 - 4.0 tons of cooling potential each summer night if the roof night sky radiation could
be effectively captured. In many North American locations, the available nocturnal cooling exceeds
the nighttime cooling loads and in arid desert climates may be considerably in excess of total daily
cooling requirements. However, various physical limitations (differential approach temperature, fan
power, convection and conductance) limits what can be actually achieved, so that perhaps half of
this cooling rate can be practically obtained. Even so, careful examination of vapor compression
space cooling in many homes in Florida shows that typical homes experience cooling loads
averaging 33 kWh per day from June - September with roughly 9.2 kWh (28%) of this air
conditioning coming between the hours of 9 PM and 7 AM when night sky radiation could greatly
reduce space cooling.
A big problem with previous night sky radiation cooling concepts have been that they have typically
required exotic building configurations. These have included very expensive “roof ponds” or, at the
very least, movable roof insulation with massive roofs so that heat is not gained during daytime
hours. To address these limitations, an innovative residential night cooling system is described as
shown in Figure E-1. The key element of this configuration is that rather than using movable
insulation with a massive roof or roof ponds, the insulation is installed conventionally on the ceiling.
The system utilizes a metal roof over a sealed attic with an integrated dehumidification system.
2
Figure E-1. Schematic of NightCool concept.
1.
2.
3.
4.
5.
6.
White metal roof on metal battens (no decking). Both sides are
surfaced for high emissivity. A temperature probe measures roof
underside temperature.
Small capacity dehumidifer (such as Whirlpool AD40DBK); operates
only during evening hours when thermostat and roof temperature
monitor calls for cooling and attic relative humidity is greater than
55%.
Baffled inlet frill from attic for nighttime operation.
Room return inlet (for daytime operation). Closed by damper at
night when temperature conditions are met.
Thermostat (compares roof surface temperature and setting to
determine vapor compression vs. nighttime cooling operation).
Variable speed air handler fan with electronically commutated
motor.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
Vapor compression air conditioner cooling coil.
Interior duct system with supply outlet.
Interior room air return to attic during evening hours when Night
Cool is activated.
Roofline drip collection system with drain.
Ceiling return for NightCool operation mode.
Attic air connects to cool roof for nocturnal cooling.
R-20 ceiling insulation.
Sealed attic construction with top plate baffles (tested and sealed
system).
Air conditioner outdoor unit (condenser).
Concrete interior walls (thermal mass for sensible cool storage).
Tile floor (add thermal mass).
During the day, the building is de-coupled from the roof and heat gain to the attic space is minimized
by the white reflective metal roof. During this time the space is conventionally cooled with a small
air conditioner. However, at night as the interior surface of the metal roof in the attic space falls two
degrees below the desired interior thermostat set-point, the return air for the air conditioner is
channeled through the attic space by way of electrically controlled louvers with a low power
variable speed fan. The warm air from the interior then goes to the attic and warms the interior side
of the metal roof which then radiates the heat away to the night sky. As increased cooling is
required, the air handler fan speed is increased. If the interior air temperature does not cool
sufficiently or the relative humidity is not kept within bounds (<55% RH) the compressor is
energized to supplement the sky radiation cooling. The massive construction of interior tile floors
and concrete walls) will store sensible cooling to reduce daytime space conditioning needs. The
concept may also be able to help with daytime heating needs in cold climates as well by using a
darker roof as a solar collector.
3
Within the report, we create a simulation model of the NightCool cooling system to examine the
fundamental relationships influencing performance. The computer simulation extends work by
Givoni and others. The calculations evaluate performance based on physical principles. The
simulation has been used to evaluate the impact of various physical system parameters (roof
conductivity, view factors, system air flow rate etc) as well as various system values (return air
temperatures and flow rates) as well as weather conditions. In this way, it is possible to determine
the critical system parameters to improve system performance. As example, Figure E-2 below shows
the NightCool model sensitivity to the system air flow rates.
Figure E-2. Impact of NightCool system airflow on predicted cooling.
Using TMY weather data, the simulation shows that in Tampa, Florida from June - September, the
system can produce an average of 15 kWh of cooling per day at a use in fan power of about 1.4 kWh
for a system EER of about 37 Btu/Wh – considerably more efficient than conventional vapor
compression air conditioning equipment. As expected, predicted performance in less humid and
more temperate climates showed even greater potential. Phoenix, Arizona demonstrated a daily
average summer cooling potential of 23 kWh at a SEER of 61 Btu/Wh. Performance in Atlanta,
Georgia and Baltimore, Maryland was even better – 50 and 62 kWh of daily cooling available at
efficiencies greater than 70 Btu/Wh.
The empirical evaluation of the concept will be accomplished by using two highly instrumented
side-by-side 10 x 16' test sheds located at the Florida Solar Energy Center. One of the test sheds will
be configured like a conventional home with a dark shingle roof and insulated ceiling under a
ventilated attic. The experimental shed will feature a white reflective roof on battens with a sealed
attic where the air from the shed interior can be linked to the sealed attic and roof radiator when the
roof temperature drops below the room target cooling temperature.
4
Theoretical Evaluation of the NightCool Nocturnal Radiation Cooling Concept
Danny S. Parker
April 2005
Introduction
Using a building’s roof to take advantage of radiation to the night sky as a heat sink has been long
identified as a potentially productive means to reduce space cooling. Radiative cooling to the night
sky is based on the principle of heat loss by long-wave radiation from one surface to another body
at a lower temperature (Martin and Berdahl, 1984). In the case of buildings, the cooled surfaces are
those of the building shell and the heat sink is the sky since the sky temperature is lower than the
temperature of most earth bound objects.
Theoretical Potential
The night cooling resource is large and enticing for residential energy-efficiency applications. On
a clear desert night, a typical sky-facing surface at 80EF (27EC) will cool at a rate of about 75 W/m2.
In a humid climate with the greater atmospheric moisture, the rate drops to about 60 W/m2. Nighttime cloud cover is an important variable as well. With 50% cloud cover in a humid climate, the
cooling rate drops to about 40 W/m2 and only about 7 W/m2 under completely overcast skies.
Average potential daily July cooling for a radiator at 72EF amounts to 63 - 110 Wh/m2 of roof
surface in U.S. climate locations – the lower value being representative of a humid region like that
in Florida (Clark, 1981). For a standard sized U.S. home (200 m2 of roof area), this amounts to 13 220 kWh of available daily cooling if the roof night sky radiation could be effectively captured. In
many North American locations, the available nocturnal cooling exceeds the nighttime cooling loads
and in arid desert climates may be considerably in excess of total daily cooling requirements.
Over a 10 hour night, this amounts to about 250 - 450 W/m2 if all could be effectively utilized.
However, that is not easily realized. Winds add heat to the roof by convection and thus reduce
beneficial heat transfer from night sky radiation. Under an average wind speed of 2 mph (0.9 m/s)
– the potential diminishes by about half of the above. Also, water condensation – dew – also limits
the temperature depression we can achieve for exposed surfaces.1
Only a portion of the potential cooling can be obtained since the heat must be transferred from the
building to the radiator and then to the sky. The rest will cool the radiator down until it gains more
heat from surrounding air or reaches the dew point and is effectively lost for cooling purposes.
Various physical limitations (differential approach temperature, fan power, convection and roof
conductance) limits what can be achieved, so that perhaps half of this rate of cooling can be
practically obtained. Even so, careful examination of air conditioner operation in many homes in
Florida (Parker, 2002) shows that typical residences experience cooling loads averaging 33 kWh per
1
There is some degree of sub-cooling below the dew point, but for the purposes of discussion this can be generally dismissed.
5
day from June - September with roughly 9.2 kWh (28%) of this air conditioning coming between
the hours of 9 PM and 7 AM when night sky radiation could substantially reduce cooling needs.
Trade-offs depend on many factors such as the interior convection from the radiating surface to the
building as well a the fan power used to capture the cooling resource. A detailed calculation routine
is provided in Appendix A evaluating these factors. However, passive systems with very little air
velocity under the radiator (or those with a small circulation fan) still will achieve delivered net
cooling rates of 1 - 5 W/m2. With 200 m2 of roof in a typical home that adds up to a nearly free
cooling rate of 200 - 1,000 Watts (700 - 3,400 Btu/hr). Systems with higher air flow rates (800 cfm
or 1,360 m3/hr), can achieve net cooling rates about twice that level.
Extensive work has examined the use of exotic night cooling schemes: roof ponds, massive roofs
with moveable insulation, combined desiccant and radiative cycles and other technologies (Hay,
1978; Givoni, 1994).1 Often, however, issues such as operational complexity and parasitic fan power
have made them unpromising.
Empirical Data on Potential of Nocturnal Cooling
Given the complexity of the physical phenomenon, to further evaluate the potential of night cooling,
we realized we could use available roof surface temperature data recorded over several years at an
FSEC roof test facility (Figure 1).
The plot shown in Figure 2 shows data taken from FSEC's Flexible Roof Facility from the summer
of 2001 which graphically illustrates the potential of the night cooling concept. The plotted
temperature shows the 15-minute average daily roof surface temperatures of the weathered white
metal roof underside temperature in test cell #6 (see Figure 1) over as compared with the outdoor
air temperature and an indoor temperature target of 77EF. Within evaluation of night cooling
concepts, the temperature shown is termed the stagnation temperature – the temperature a surface
will drop to without any heat being added to the surface.
6
Figure 1. White standing seam metal roof on the right at the Flexible Roof
Facility (FRF) used to evaluate NightCool potential.
Figure 2. Flexible Roof Facility average metal roof temperature depression in the
summer of June - September 2001.
The nighttime temperature depression of the roof surface averages 3-4EF below the aspirated air
temperature. Average depression from 9 PM to 7 AM was 3.0EF. The minimum roof surface
temperature is limited by the dewpoint temperature. When the surface falls below the outdoor
dewpoint temperature (typically between 72E and 74EF during summer) heat is added to the roof via
this physical mechanism. Thus, the dewpoint temperature exacts a real constraint to the night sky
cooling potential.
7
Using this measured data, a series of assumptions allows a crude evaluation of the night cooling
potentials. Assuming a 77EF interior setpoint, the roof surface temperature provides a temperature
sink which averages 6.5EF cooler than the interior target temperature between 9 PM and 7 AM.
Even assuming that there was only still air convection between the main zone and the outside night
sky (Hō ~1.6 Btu/ft2/EF), this equates to about 10 Btu/ft2/hr during the average night (ASHRAE,
2001). For a 2,000 ft2 roof, that would indicate about 20,000 Btu/hr or 5,860 W (1.67 tons) of
nightsky cooling, although only a portion of the cooling potential could be realized as constrained
by convection within the attic space and the associated heat exchange process. However, this rate
is in general agreement with the previously described estimates (Fairey, et. al., 1986).
On the other hand, the realistic cooling rate would be helped as the roof stagnation temperature
depression (which is shown above in Figure 1) includes long period of where dew is adding heat to
the roof which would be warmed if in communication with the indoors below (less dew). Also, the
rate of radiation to the sky would be increased (to the 4th power of the Stefan-Boltzmann constant)
as the roof surface temperature rose from heat transfer to the roof, increasing the cooling rate.
Finally, the scheme could have some forced convection in the attic zone so that the convective
surface heat transfer coefficient (Hō) would likely be in the realm of 3-4 Btu/ft2/EF. Of course these
represent average conditions so some nights would be better than others.
Description of the NightCool Concept
We propose an innovative night cooling system consisting of a metal roof serving as a large area,
low mass highly conductive radiator (see Figure 3). The metal roof could be used at night during
fall, autumn and acceptable summer periods to perform sensible cooling. It could also be used for
heating during winter daytime operation where heating of the metal roof could be used to heat the
home during midday and late afternoon hours when weather conditions were beneficial.
A recurring problem with night sky radiation cooling concepts have been that they have typically
required exotic building configurations. These have included very expensive “roof ponds” or, at the
very least, movable roof insulation with massive roofs so that heat is not gained during daytime
hours (Hay, 1978; Fairey, et. al., 1990). The key element of the described configuration is that rather
than using movable insulation with a massive roof or roof ponds, the insulation is installed
conventionally on the ceiling. The operation of the system is detailed in the attached schematic.
8
Figure 3. Schematic of NightCool concept.
1.
2.
3.
4.
5.
6.
White metal roof on metal battens (no decking). Both sides are
surfaced for high emissivity. A temperature probe measures roof
underside temperature.
Small capacity dehumidifer (such as Whirlpool AD40DBK); operates
only during evening hours when thermostat and roof temperature
monitor calls for cooling and attic relative humidity is greater than
55%.
Baffled inlet frill from attic for nighttime operation.
Room return inlet (for daytime operation). Closed by damper at
night when temperature conditions are met.
Thermostat (compares roof surface temperature and setting to
determine vapor compression vs. nighttime cooling operation).
Variable speed air handler fan with electronically commutated
motor.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
Vapor compression air conditioner cooling coil.
Interior duct system with supply outlet.
Interior room air return to attic during evening hours when Night
Cool is activated.
Roofline drip collection system with drain.
Ceiling return for NightCool operation mode.
Attic air connects to cool roof for nocturnal cooling.
R-20 ceiling insulation.
Sealed attic construction with top plate baffles (tested and sealed
system).
Air conditioner outdoor unit (condenser).
Concrete interior walls (thermal mass for sensible cool storage).
Tile floor (add thermal mass).
During the day, the building is de-coupled from the roof and heat gain to the attic space is minimized
by the white reflective metal roof. At this time the space is conventionally cooled with a down-sized
air conditioner. However, at night as the interior surface of the metal roof in the attic space falls two
degrees below the desired interior thermostat set point, the return air for the air conditioner is
channeled through the attic space by way of electrically controlled louvers with the variable speed
fan. The warm air from the interior then goes to the attic and warms the interior side of the metal
roof which then radiates the heat away to the night sky.
As increased cooling is required, the air handler fan speed is increased. If the interior air temperature
does not cool sufficiently or the relative humidity is not kept within bounds (<55% RH) the
compressor is energized to supplement the sky radiation cooling. However, by midnight on clear
nights, the temperature of the metal will have dropped sufficiently to begin to dehumidify the air
introduced to the attic. The collected moisture on the underside of the roof will then drain to
collection points at either side of the eaves so that the home can be dehumidified during evening
9
hours by way of the operation of the blower fan (200-300 W). Also, if temperature conditions are
satisfied, but relative humidity is not, a dedicated attic dehumidifer (note 2 on Figure 3) is energized.
The massive construction of the home interior (tile floor and concrete interior walls) will store
sensible cooling to reduce space conditioning needs during the following day.
Description of the Radiative Cooling Prediction Model
To objectively evaluate the potential of the NightCool concept, we created a simulation model based
on the fundamental physics involved. The resulting model is described below with the spreadsheet
contained in Appendix A.
Nocturnal cooling is governed by the radiation of all surfaces to the night sky via thermal radiation
exchange. To further understand the mechanisms so we can apply them, we summarize the various
calculations involved.
The radiation between a radiating surface and the night sky can be described by approximating the
surface as a black body. The heat loss to the sky by radiation can be estimated as:
Qrad = 0r(SB Trad4 - Tsky4)
Where:
Qrad
0r
SB
Trad
Tsky
=
=
=
=
=
radiation heat flux to sky (W/m2)
long-wave emissivity of the radiating surface
Stefan-Boltzmann constant (5.67 x 10 -8 W/m2 K)
absolute temperature of the radiating surface (K)
effective sky temperature (K)
A key parameter in the above equation is the effective sky temperature.2 This depends on a number
of factors, but strongly on outdoor temperature, cloud cover and atmospheric moisture content. The
impact of cloud on radiation is complex, but is estimated by Clark et al. (1981, 1982) to vary so that
the net radiation from a cloudy sky can be roughly approximated by percent of cloud cover (Clark
and Blanplied, 1979):
Ca
= 1.00 +0.00224*n + 0.0035n2 + 0.0028n3
Where:
n
= opaque sky cover in tenths
Thus, on average, even a cloudy sky has about 45% of the sky radiation potential of a clear sky.
However, moisture also plays a major role so that humid locations see a higher effective sky
emissivity and thus, less radiative cooling potential. Often these two go together– cloudy skies with
2
The sky temperature is useful construct that it provides a single value which quantifies the complex infrared emission properties of the various
gases in the atmosphere. It is defined as the temperature of a blackbody object emitting the same amount of thermal energy as the sky.
10
moist atmospheric conditions. The clear sky emissivity can be estimated by a correlation developed
by Berdahl and Martin (1984):
0Clear = 0.711 + 0.56(Tdp/100) + 0.73 (Tdp/100)2
Where:
Tdp
= dewpoint temperature in degrees Centigrade
The effective clear sky temperature is then:
Tclear sky = Ta (0Clear0.25)
Where:
Ta
=
absolute ambient air temperature (K)
The effective temperature of the cloudy sky is then:
= (Ca)0.25 * Tclear sky
Tcloudy
Predicting Roof/Attic Radiator Heat Exchanger Performance
To evaluate the performance of the night cooling system, it is necessary to calculate the temperature
of the heat transfer to the air flowing through a one-dimensional path across the radiator as described
by Santamouris and Asimakopolous (1996). This calculation is analogous to that used to evaluate
heat transfer for solar collectors that use air as the heat transfer medium:
Tout - Tth = (Tfi- Tth) exp (-UpA/mcp)
Where:
Tout
Tth
A
Up
mcp
=
=
=
=
=
outlet temperature of the air from the radiator (c)
inlet temperature of the air to the radiator (c)
surface area of the radiator (m2)
overall heat transfer coefficient with air circulating through the radiator (W/m2-k)
air mass density product
The heat transfer coefficient (Up) from the radiator to the circulating air is then:
Up
=
1
(1/he) + (1/hf) + (drad /krad)
11
Where:
he
hf
drad
krad
=
=
=
=
effective combined heat transfer coefficient (W/m2-k)
convective heat transfer coefficient between radiator surface and the air (W/m2-k)
thickness of the radiator surface (m)
thermal conductivity of the radiator surface (W/m2-k)
The interior roof surface convective heat transfer coefficient is very important within the solution
of the equations as convection remains a major means to transfer heat to the roof radiator:
hn,rd,u =
9.624 (Tsurf - Tair)0.33
7.333 - cos (β)
Where
hn
Tsurf
Tair
β
=
=
=
=
convective heat transfer coefficients (W/m2@K)
temperature of component surface (K)
temperature of air (K)
roof slope pitch angle (degrees)
Although air movement in attics is generally assumed to be dominated by natural convection, fan
powered air circulation may increase surface heat transfer due to forced convection. For instance,
Metais and Eckert (1964) demonstrated that a mixed convection regime between turbulent and
laminar flow regimes will tend to increase free convection over the assumption of uniform turbulent
flow. In order to accommodate potential forced convection, a forced convection term was added for
attic surfaces to estimate its magnitude.
Peavy (1979) recommends the following relationship for turbulent flow over smooth surfaces at low
air velocities (V < 3 m/s):
hf
= 7.176 (Va)0.8 L-0.2
Where:
hf
Va
L
= forced air convection coefficient ( W/m2-K)
= air velocity over surface (m/s)
= air flow path length (average length of the radiator surface, m)
The air flow path length is taken as the sum of the eave to ridge vent distance and that attic plenum
height. The calculation of average air velocity over attic surfaces is problematic since interior attic
air flows are complex. Ford (1982) estimates attic air flow velocity (m/s) based on the volumetric
rate of ventilation over the cross sectional area of the attic perpendicular to the direction of flow:
Va
=
Flowsec (Vola)
Afp
12
Where:
Flowsec = volumetric flow rate (m3/sec)
= attic volume (m3)
Vola
Afp
= cross sectional area of air flow path (m2)
Unfortunately, such a calculation greatly underestimates the actual air velocities proximate to the
attic heat transfer surfaces, particularly for the roof deck, since air flow through the attic is nonuniform. Unfortunately without more involved CFD fluid modeling of the attic space under the
forced fan flow that would be anticipated in the operating system, the specific velocities remain an
uncertainty within the model.
Based on trial and error using the model, we found that the most satisfactory solution was to take
the maximum value of the solution of two equations, one for still air or low air velocities and another
for forced flow regimes.
Roof Convective Heat Transfer to Outdoors
On the other side of the radiator heat balance, we need to estimate the convective heat transfer to
the roof to outdoors. As shown in numerous studies, this factor has an important impact on radiative
cooling systems. The convective heat transfer coefficient of the outside roof surface was
approximated by assuming fully developed turbulent flow across the entire roof section. Relatively
few studies have been completed on measured convection heat transfer for building exterior
surfaces. However, we take a simple correlation is taken from Burch and Luna (1980):
hr
= 2.8 + 4.8 (V')
Where:
hr
V'
= roof convective heat transfer coefficient (W/m2@K)
= wind speed parallel to the roof surface (m/s)
A reduced wind speed at the roof surface was first determined using the correlation recommended
by Sherman and Modera (1986) to translate weather tower wind speeds to those on site for suburban
residential construction:
V'
α,w (Hw/10)γ,w
= ))))))))))
)) * V
α,s (Hs/10)γ,s
Where:
α,w = alpha terrain parameter for weather tower (0.67)
Hw = height of wind tower (m) (10)
γ,w = gamma terrain parameter for weather tower (0.15)
13
α,s
Hs
γ,s
V
=
=
=
=
alpha terrain parameter for site (1.0)
height of site (m) (3.0)
gamma terrain parameter for site (0.25)
wind speed at measurement site (m/s)
This relationship would suggest that the rooftop estimated wind speeds for a suburban environment
(Shielding and Terrain Class IV) would be roughly 55% of the wind speed velocities measured at
10 m at a standard weather station. However, we found this to be pessimistic relative to actual
rooftop velocity measurements made at FSEC’s FRF facility which found wind speeds to only
average 15% of the values measured at a 10 meter height at the same facility (Parker and Sherwin,
1998). This is very likely due to the influence of the roof structure itself in reducing boundary layer
air velocities. As such, we used a value of 15% of the measured 10 meter wind speeds as an input
to the model. This assumption will be an important one to validate within coming monitoring.
Weather Data and Assumptions
We chose to use hourly TMY2 weather data with the model to show indicated performance. Within
our base case analysis, we used data for Tampa, Florida to look at performance indicators in a
disadvantageous climate. The other critical input parameter is the air temperature at the inlet to the
radiator. Within the modeling, we assumed that this temperature was 26EC (78.8EF) – likely
representing a room temperature being maintained of 77EF (25EC). The higher assumed inlet
temperature is due to internal gains from people, solar heating and appliances as well as the
Nightcool circulation fan heat.
Roof Radiator Surface Temperature Balance
The predicted roof surface temperature is estimated as an energy balance taking into account the roof
surface emittance properties, view factor, the sky temperature and the associated radiative,
conductive and convective heat transfer coefficients. The net heat flux (Qr in Watts) of the roof
radiator at a temperature (Tr) is a linear function of the effective heat transfer coefficient (he) and the
stagnation temperature (Tth).
Qr
= he (Tr- Tth)
he is then:
he
= hr + 4 σ Ta3
Where:
hr
Tth
Th’
= the roof convective heat transfer coefficient previously solved (W/m2-k)
= the minimum stagnation temperature the radiator can achieve, constrained to be no less
than 1 degree centigrade below the ambient dewpoint temperature (EC)
= Ta - q0/ he
14
Th
= Max |Th’, Tdew-1|
And the net radiative power (Watts) of a black body at the ambient temperature (Ta) is produced by:
q0
= σ Ta4 - qs
And:
qs
= σ ε sky Ta4
Where:
Ta = ambient air temperature (K)
σ
= Stefan Boltzmann constant
ε sky = sky emissivity as modified by cloudiness (dimensionless)
The air temperature at the outlet of the radiator is then determined from previously defined terms:
Tout = Tth + (Tfi- Tth) exp (-UpA/mcp)
The final result is based on the calculated system air flow rate, change in inlet to outlet temperature
less the fan heat supplied to the system:
Qwatts = (1220 * Flow (m3/hr) * (Tfi- Tout)/3600) - Fanwatts
Qwatts is the hourly cooling rate produced by the system. The standard flow rate used with the model
was 1360 m3/hr (800 cfm). Fan power depends on the assumed fan efficiency (40%), fan and duct
static pressure (0.6 IWC or 150 pa) and motor efficiency (60%). The base calculation assumes that
fan power is approximately 235 Watts to produce the base flow rate.
Using the baseline parameters given in Table 1 with the described equations, the model predicts a
whole house cooling rate of 2,157 watts, or just over half a ton of night cooling. Below we evaluate
model sensitivity to the large range of parameters that influence performance.
Dewpoint
Generally, the clear sky emissivity varies with the dew point temperature declining as dew point
temperatures fall. This obviously varies with climate. One commonly described index to radiative
cooling potential is the “sky temperature depression” which is merely the average difference
between the air and sky temperature. The nightly summer average sky temperature depression varies
between 5 and 10EC (9-18EF) around the U.S. Generally, dry locations have the greatest sky
temperature depression – and thus the greatest potential to take advantage of radiative cooling.
Figure 4 (Clark, 1981) shows a map of how the sky temperature depression average for July around
the United States with a roof surface temperature of 74EF.
15
Figure 4. Available night sky cooling (Btu/day/ft2) around the U.S at a 74EF radiator temperature (Clark, 1981).
By using the model with base case simulation values shown in Table 1, we were able to evaluate
sensitivity to outdoor temperature and dewpoint as shown in Figures 5 and 6 and Tables 2 and 3.
Note the large sensitivity to outdoor temperature.
Table 1
Base Case Simulation Values for Analysis
2,400 ft2 Roof
Parameters
Radiator Thickness (metal roof)
Length of Radiator (roof length)
Width of Radiator (roof width)
Height of Duct (attic space)
Thermal Conductivity (metal conductivity)
Radiator Emissivity (roof emissivity)
View Factor to Sky (dimensionless)
Air Velocity under Radiator (inside attic)
Ambient Temperature (EC)
Stefan-Boltzmann Constant
Wind Velocity (w/ss)
Average Cloudiness (0-10)
Air Temperature at Inlet (EC)
Dewpoint (EC)
Air Flow (in 3/hr through plenum)
Roof Tilt Angle (4/12 tilt)
Fan Efficiency (fractional)
Static Pressure (fan pressure)
Motor Efficiency (PSC motor)
16
Standard Values
0.002 m
15 m
15 m
1m
16 W m-1 K
0.88
0.98
0.03 m/s
24EC
5.67E-08
2 m/s
5
27EC
22EC
1,360 m3/hr
18.4 degrees
0.4 (40%)
144 Pa (0.6 IWC)
0.6 (50%)
Figure 5. Sensitivity of predicted night sky estimated cooling rate vs. outoor ambient
temperature for a radiator inlet temperature of 27EC.
Table 2
Influence of Outdoor Air Temperature on Predicted Cooling Rate of NightCool Concept
Ambient Temperature
(EC)1
Watts
20
21
22
23
24
25
26
27
28
29
30
4,058
3,580
3,101
2,620
2,138
1,653
1,165
675
188
0
0
1
Dewpoint = Tdrybulb-2
17
Figure 6. Sensitivity of predicted night sky cooling rate vs. outdoor dewpoint.
Table 3
Influence of Outdoor Dewpoint on Predicted Cooling Rate of NightCool Concept
Tdp1
(EC)
Watts
18
19
20
21
22
23
24
2,241
2,215
2,189
2,164
2,137
2,029
1,595
1
Ta = 24EC
Emissivity and Spectral Characteristics
Building surfaces radiate to the night sky primarily in the so-called “sky window” between 8 and
13 nanometers. Thus surfaces which have their highest emittance in this range will perform best.
Also, to reduce daytime heat gain, materials which are highly reflective in the short wave range
(solar spectrum) will also perform best. This will also promote radiant cooling during daytime hours,
as well as reflect incoming atmospheric radiation form water, dust and carbon dioxide. Best
performing are white paints or other light colors with strong reflectivity in the short wave range.
18
Fortunately, most common building materials, other than foils or metallic paints have relatively high
long-wave emissivities. These are generally in the range of 0.85 - 0.90. There are, however, some
special coatings such as anodized aluminum surfaces which provide slightly greater performance,
although a thorough literature review by Givoni, revealed little real efficiency gain and the strong
likelihood that all surfaces would be similar once condensation has formed on the surface.2 In
general however, such surfaces are exotic and are impractical. Non-metallic light colored surfaces
will generally work well. Table 4 and Figure 7 below show the sensitivity of roof infrared emittance
to night sky cooling potential.
Table 4
Influence of Emissivity on Predicted Cooling Rate of NightCool Concept
Emissivity
(dimensionless)
Cooling
Watts
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
2,157
2,057
1,949
1,832
1,706
1,570
1,425
1,270
Figure 7. Effect of roof surface long-wave emissivity on predicted night sky cooling.
19
View Factor
The extent that the roof “sees” the sky for radiative heat exchange depends on the relative angle of
the emitter (roof) and absorber (the night sky). Generally, flatter surfaces will have the greatest
exposure to the sky for the purposes of radiation, while those vertically inclined will not. Within
everyday experience, this is the reason that frost may form on the windshield of a car, but will be
much less on the vertical side windows. This factor is related to the geometry of the roof relative
to the sky and is referred to as the “view factor.” This is potentially important, since radiation is a
line-of-sight phenomenon, the orientation of the building surface has a lot to do with how it interacts
with nocturnal radiation. Generally, the lower the roof slope, the stronger the radiation linkage to
the night sky since the roof views more of the sky than the ground and local surroundings. The
coldest temperatures are to be found up in the sky dome at the zenith. Then too, strongly sloped roof
surfaces (5/12 or greater) generally see just about as much of the radiation from the much warmer
earth and surrounding buildings and landscape, as they do from the cooler sky.
The mathematics of the view factor can be physically described for surfaces, but those looking to
take advantage of the benefits of night sky radiation can simply keep in mind that lower slope roofs
will work best. A simple rule of thumb is that a vertical wall with an unobstructed view (no trees or
adjacent buildings) will see about a third of the sky as will a horizontal roof. The view factor for
different oriented planes can be geometrically complex. However, for the simple case of a roof
exposed to the sky, this “view factor” reduces to:
VF =
1+ cos ( β)
2
Where:
VF = the dimensionless view factor that modifies radiative heat exchange
β
= the roof tilt angle (degrees)
Since the roof tile angle relative to the sky is zero for a flat roof, the view factor is unity (1.0).
However, common roof tilts don’t tend to reduce the view factor to a large extent. For instance, a
common 5/12 roof pitch only results in a roof tile of 22.6 degrees, with a view factor of 0.965– a
reduction of the potential night sky radiation of only 3.5%. Of course at very high tilts, the view
factor is adversely affect (for instance a vertical surface has only about half the potential irradiance
of a horizontal surface). However, impacts are limited for virtually, all typical tilt angles for
common roof pitches.
Also, there are compensating physical factors. Practically, as roof pitch increases, in real roofs, the
roof surface area also increases, largely compensating for the reduction to view factor. Table 5 and
Figure 8 below show that the model indicates that roof irradiance does not vary significantly with
roof pitch largely due to these compensating factors.
20
Table 5
Influence of Roof Tilt and View Factor on Predicted Cooling Rate
Roof Pitch
(slope/degrees)
Watts
0/12 (0o)
1/12 (4.8o)
2/12 (9.5o)
3/12 (14.0o)
4/12 (18.4o)
5/12 (22.6o)
6/12 (26.6o)
7/12 (30.3o)
8/12 (33.7o)
2,157
2,153
2,146
2,143
2,138
2,123
2,115
2,104
2,088
Figure 8. Effect of roof tilt and view factor on night sky cooling rate.
Wind
Night sky cooling can be strongly affected by winds. As building surfaces fall below the ambient
air temperature, they begin to gain heat by air convection. Still air and windless conditions produces
the least heat added to the exposed surface. Thus anything that reduces wind speeds around the
radiating surfaces (without blocking the sky view) will help increase nocturnal radiation. Generally,
in most developed areas this is not much trouble since, wind speeds in suburban developments at
21
rooftop level tend to average about 10-20% of those recorded at nearby airports (Parker and
Sherwin, 1998).
For instance, an 80EF roof radiating to the clear night sky at a rate of 60 Watts/m2 in a humid climate
will gain about 20 W/m2 of heat from surrounding still air as it cools below the air temperature
leaving about 40 W/m2 potentially available for cooling. However, the same roof with a wind speed
at the roof line of 5 mph (2.25 m/s) will gain about 43 W/m2 from the surrounding air leaving less
than half of the cooling potential!3
However, wind is a weather factor that not much can be done about unless sophisticated schemes
are considered. Ideas include the use of lightweight polyethylene films suspended above the
radiators which are transparent to long wave radiation, but reduce heat gain from air movement.4
This is generally impractical. Again, Givoni (1994) has done some intriguing experiments verifying
the added potential under various conditions. Whereas an exposed radiator under dry clear
conditions was able to reach a depressed temperature about 7EC lower than ambient, the screened
surface reached 9EC lower. Under cloudy skies, the temperature depressions were about 5EC and
6.5EC, respectfully. However, for humid conditions, the performance of an exposed radiator may
be similar to a screened one once dew forms on the polyethylene surface. Also, the fundamental
impracticality of screen radiators remains so that only uncovered radiator systems really seem
practical. Figure 9 shows the model sensitivity to outdoor wind velocity.
Figure 9. Sensitivity of predicted night sky estimated cooling rate vs. outdoor wind speed.
22
Table 6
Influence of Windspeed on Predicted Cooling Rate of NightCool Concept
Windspeed
(m/s)
Watts
0
1
2
3
4
5
2,417
2,293
2,138
1,941
1,808
1,712
Cloudiness
A cloudy sky during nighttime hours is a known factor in reducing the potential for nocturnal
cooling – a fact evident from the fact that frost or fog most readily occur following a clear night, but
not often following those that are overcast. Clouds, particularly, low stratus and cumulus clouds,
emit radiation through the entire long-wave spectrum so that under a completely overcast sky,
nocturnal radiation is largely eliminated. On the other hand, clear sky conditions allows night sky
radiation to reach its maximum potential, although also influenced by atmospheric moisture content.
Table 7 and Figure 10 below show how sky cloudiness influences the cooling potential of the night
sky concept. Note that a completely overcast sky causes the potential to drop to approximately half
of the clear sky value. The value does not drop to zero only because the outdoor ambient air
temperature (24EC or 75.2EF) is less than the assumed entering temperature for the roof radiator
(27EC or 20.6). However, if the entering temperature dropped to 75EF, there would no longer be any
useful radiational cooling from the concept.
Table 7
Influence of Cloudiness on Predicted Cooling Rate of NightCool Concept
Cloudiness
(Index: 1-10)
Watts
0
1
2
3
4
5
6
7
8
9
10
2,461
2,459
2,348
2,267
2,201
2,138
2,063
1,964
1,826
1,637
1,379
23
Figure 10. Impact of cloudiness on predicted night sky cooling rate.
Return Air Temperature
The temperature maintained inside the house and the return air temperature brought to the attic
radiator can be expected to have a very large influence on the cooling potential. Figure 11 below
illustrates this graphically. The cooling rate drops quickly as the temperature of the roof radiator is
approached. Realistically, temperatures below 74EF under the base case constraints cannot achieve
significant night sky cooling. However, the return air temperature is always slightly above the room
temperature due to added internal gains, left over solar gains and fan heat. Given the large influence
of this system operating characteristic, it becomes important to keep the return air temperature in
the attic temperature as warm as possible. This means that it is important that fan and dehumidifier
heat be introduced to the plenum space so that it can be readily radiated away.
24
Figure 11. Impact of Return Air Temperature on predicted night sky cooling rate.
Table 8
Influence of Return Air Inlet Air Temperature on Predicted Cooling Rate
of NightCool Concept
Return Air
Temperature
(EC)
Watts
24
25
26
27
28
29
720
1,195
1,667
2,138
2,605
3,072
25
Return Air Flow Rate
The rate at which air flow is introduced to the attic plenum exacts a major impact on the cooling
potential on the night sky concept. Not only does the air flow rate add heat to the roof radiator, but
it also increases convective heat transfer from the attic to the roof surface with increased air
velocities. However, as the flow rate is increased, the energy for the circulation fan operation goes
up at the cube of the air flow. Thus, fan heat goes up with the flow rates. Note from Figure 12 that
cooling rate increases rapidly as the flow rate is increased, but begins to level off as flow goes
beyond 2,000 cfm. Given the large influence of system air flow rates, the cooling rate of the concept
could most easily be adjusted by varying the circulation air flow through the use of a variable speed
fan and brush-less DC motor combination (variable speed air handler). Conventional variable speed
air handlers are capable of the rates indicated for the concept.
Fig 12. Impact of attic plenum air flow on predicted night sky cooling rate.
26
Table 9
Influence of Air Flow Rate on Predicted Cooling Rate of NightCool Concept
Air Flow Rate
(cfm)
Watts
0
500
1,000
1,500
2,000
2,500
3,000
3,500
0
858
1,644
2,308
2,807
3,136
3,307
3,338
Fan Motor Efficiency
The efficiency of the fan motor directly impacts the required electrical energy for the operation of
the night cooling system and thus, the overall system COP. The waste heat of the motor also reduces
the performance of the cooling cycle. Figure 13 below shows the limited influence of circulation fan
motor efficiency on delivered cooling. Three common types of motors are shown: command
permanent split capacitor (PSC) motors (about 60% efficient), higher efficiency PSC motors (65%
efficient) and the brush-less DC electronically commutated (ECM) motors which feature the highest
efficiency.
Realistically, the concept would require the ECM motors as the air flow rate would almost certainly
be used to throttle the night sky cooling rate. Also, the efficiency of PSC motors fall off dramatically
if capacitors are used to actinically lower fan. The ECM motors do not suffer this loss in efficiency.
Also, even at full speed these motors also have the advantage of being about 85% efficient.
Although the fan/motor efficiency only has a modest impact on the absolute delivered space cooling
(Watts) shown in Figure 13, its influence on the efficiency with which cooling is delivered is
dramatic. This is because, the fan and motor power is the overall electrical energy consumption of
the cooling system and thus influences the comparative cooling efficiency. For conventional vapor
compression air conditioning systems, this is rendered as the energy efficiency ratio (EER) or the
seasonal energy efficiency ratio (SEER) which reflects cooling performance over the year. EER is
given as Btu/Watt hour of electricity use (3.413 * the system COP). Conventional air conditioning
systems have EERs of 9 -15 Btu/Wh.
Figure 14 shows that while the NightCool system delivers cooling at greater than 31 Btu/Wh with
a PSC motor under the reference conditions, the efficiency is boosted by nearly 50% to over 45
Btu/Wh with an ECM motor. The impact is even larger than shown if PSC motors must have speeds
modulated. When operating at lower than rated speeds, the fractional efficiency of PSC motors can
decrease to 45% or less-- much lower shown in both graphs. As achieved cooling rates are most
readily modulated by altering NightCool system air flow (see Figure 12), this emphasizes how the
27
use of variable speed ECM motors are almost a necessity for successful implementation of the
concept.
Table 10
Influence of Motor Efficiency on Predicted Cooling Rate of NightCool Concept
Motor
(Fractional Efficiency)
Watts
0.6
0.65
0.85
2,138
2,156
2,207
Fig 13. Impact of circulation fan efficiency on predicted night sky cooling rate.
28
50
ECM Motor
EER (Btu/Wh)
45
40
35
30
Efficient
PSC Motor
Standard
PSC Motor
25
20
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
Fan Motor Efficiency (Fractional)
Figure 14. Influence of fan and motor efficiency on the predicted NightCool operating
Energy Efficiency Ratio (EER).
Analysis of Cooling Potential Using Simulation Model
Once the simulation model was created and validated against known solutions (Givoni, 1994 and
Santamouris and Asimakopolous, 1996), the model was then mated to TMY2 hourly weather data
in an Excel macro to predict performance around the year under realistically changing weather
conditions. For the calculations we use Tampa, Florida TMY2 data adjusting the weather data wind
speed to account for the greatly diminished velocity seen over roof tops in experiments done at the
FRF (Parker and Sherwin, 1998). Florida weather is less advantageous for the analysis than many
other locations since high summer dew points will often limit cooling potential. However, this
allows evaluation of the concept under difficult environmental conditions. Also, based on the
discussion above, an ECM motor was assumed to power the fan/blower assembly within the
simulation.
The resulting performance was then examined for reasonableness – eliminating daytime data under
other than low solar radiation where roof heating will negate cooling potential. Thus, the
NightCooling system was not available anytime solar irradiance exceeded 75 Btu/ft2hr (240 W/m2)
and was not activated until available cooling exceeded 1,000 Watts (3,400 Btu/hr) net of fan
operation. Even when this was done, it was discovered that the model indicated that late afternoon
and early morning hours would often show useful cooling potential during non-summer months.
Also, periods with heavy rain during summer months would sometimes activate the cooling system
for short periods. Below we show analysis for ten day periods to characterize performance in early
middle and late summer.
29
An analysis performed for early summer is shown in Figure 15 from June 1 -10 (Julian days 152162). During this ten day period the nighttime drybulb temperatures range from 67 - 82EF (average
= 73.8EF). The average nighttime dew points are 68.6EF with a cloudiness index of 3.3.
6000
20000
5500
Cooling: Average= 23.4 kWh/Day
18000
5000
Cooling Watts
4000
14000
3500
12000
3000
10000
2500
8000
Cooling Btu/hr
16000
4500
2000
6000
1500
4000
1000
2000
500
0
152
153
154
155
156
157
158
159
160
161
0
162
Julian Date
Figure 15. Hourly predicted cooling performance of NightCool system from
June 1-10th in early summer in Tampa, Florida.
Note within the plot the nightly pattern of activation of the system and cooling. In the early summer
period, the system produces 23.4 kWh/day of daily cooling operating an average of 10.6 hours per
day at a cost of 1.8 kWh of fan energy. The operating COP over this period is 13 corresponding to
an EER of 44.4 Btu/Wh. The available cooling is likely in excess of what a typical efficient home
would need for space cooling during evening hours.
A similar analysis for performance at the end of summer from October 1-10th is shown in Figure
16. During this ten day period the drybulb temperatures range from 58 - 78EF (average = 69.9EF)
with averaged dew points of 65.5EF and a cloud index of 2.8.
30
6000
20000
5500
Cooling: Average= 37.8 kWh/Day
18000
5000
Cooling Watts
4000
14000
3500
12000
3000
10000
2500
8000
Cooling Btu/hr
16000
4500
2000
6000
1500
4000
1000
2000
500
0
274
275
276
277
278
279
280
281
282
283
0
284
Julian Date
Figure 16. Hourly predicted cooling performance of NightCool system from
October 1-10th at the end of summer in Tampa, Florida.
As expected, the more favorable weather shows much greater cooling potentials in the milder season
with 37.8 kWh of daily cooling available from 12.6 hours of daily operation at a cost of 2.1 kWh
of fan energy. The operating COP is 18 for an Energy Efficiency Ration (EER) of 61.4. Again, the
available cooling is considerably in excess of daily cooling needs. This would suggest that the actual
performance of the system could be improved by having thermal mass in the building to store heat
to be rejected during nighttime hours. I also suggest that when excess cooling is available in early
summer or early fall and swing seasons that the dehumidifier could be activated to dry the building
down to perhaps 5% below the target relative humidity (55%). In this fashion, the building could
be dried when excess cooling energy is available to provide greater energy savings and a drier home
environment.
Figure 17 shows the performance of the NightCool system during peak summer conditions from July
1 -10th. During this ten day period the nighttime drybulb temperatures ranges from 70 - 84EF
(average = 76.9EF) with average dew points of 71EF. As expected, the model shows diminished
cooling capability during the warm humid nights prevalent during July.
31
6000
20000
5500
Cooling: Average= 14.2 kWh/Day
18000
5000
Cooling Watts
4000
14000
3500
12000
3000
10000
2500
8000
Cooling Btu/hr
16000
4500
2000
6000
1500
4000
1000
2000
500
0
182
183
184
185
186
187
188
189
190
191
0
192
Julian Date
Figure 17. Hourly predicted cooling performance of NightCool system from
July 1-10th during peak of summer conditions in Tampa, Florida.
Note the low performance on July 6th and 7th (Julian day 187 and 188) where the system only
operates 2-4 hours each night. Over the ten day period, nighttime cooling averages 14.2 kWh for
the predicted 8.6 hours of daily operation against a predicted fan energy of 1.4 kWh.
The seasonal analysis for Tampa from June - September showed that the nocturnal system would
operate an average of 8.6 hours per day, producing an average of 15.2 kWh of cooling per day for
a home with a consumption of fan energy of 1.4 kWh. In a typical Florida house using 33 kWh/day
this could offset about 46% of required space cooling if all could be effectively utilized. The system
average operating COP was 10.9 for a typical energy efficiency ratio (EER) of 37.1 Btu/Wh. This
roughly corresponds to a Seasonal Energy Efficiency Ration (SEER). The average daily profile of
performance is shown in Figures 18 and 19 which show weather and system performance,
respectively.
32
10
Drybulb Temperature
Dewpoint Temperture
Cloudiness Index (0-10)
Roof Windspeed (m/s)
90
9
8
7
85
6
80
5
4
75
3
2
70
1
65
0
2
4
6
8
10
12
14
16
18
20
22
Cloudiness Index & Rooftop Windspeed (m/s)
Drybulb and Dewpoint Temperature (oF)
95
24
Hour of Day: June 1 - September 30th
Figure 18. Average summer weather from June - September in Tampa, Florida.
14000
2000
1800
Cooling: Average= 15.2 kWh/Day
12000
10000
Cooling Watts
1400
1200
8000
1000
6000
800
600
Cooling Btu/hr
1600
4000
400
2000
200
0
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hour of Day
Figure 19. Average hourly predicted cooling performance NightCool system from
June - September.
The estimated available cooling compares favorable with measured data in Florida which showed
that nighttime air conditioning cooling needs averaged 9.2 kWh. It suggest that with some
dehumidification that the NightCool cooling system could reduce cooling needs. For instance, if a
500 Watt dehumidifer was used for 3 hours nightly, the combination of fan and dehumidifier energy
33
would total 2.9 kWh against the typical AC use of 9.2 kWh– a reduction in daily AC consumption
of about 6 kWh or 19%. The potential during non-summer months or in more conducive climates
was expected to be considerably greater.
Simulation in Other Climates
To examine concept performance elsewhere, we conducted the same simulation in three additional
climates which we expected to evidence substantially different potentials. These were Atlanta,
Georgia, reflecting a more moderate cooling dominated climate, Baltimore, Maryland with a mixed
heating and cooling climate and Phoenix, Arizona with an arid, very hot climate.
Results are shown in Table 11. For comparison, performance indicated from the simulation for June
- September are provided alongside those for Tampa, Florida. We also provide the results for the
month of July in parentheses to illustrate how the cooling potential varies during the hottest
conditions in each location.
Table 11
NightCool Simulation Results for Other Climates
June - September and (July Only)
Parameter
Tampa, FL
Atlanta, GA
Baltimore, MD
Phoenix, AZ
Avg Daily Cooling kWh
Avg Hrs per Night
Fan kWh
COP
SEER (Btu/Whr)
15.2 (10.8)
8.6 (7.6)
1.4 (1.3)
10.9 (8.3)
37.1 (28.4)
50.3 (42.4)
14.3 (13.9)
2.4 (2.3)
21.0 (18.4)
71.5 (62.9)
62.4 (45.4)
14.6 (13.6)
2.4 (2.3)
26.0 (19.7)
88.7 (67.4)
23.2 (11.2)
7.9 (5.3)
1.3 (0.9)
17.8 (12.4)
60.9 (42.5)
Note that each climate other than Tampa shows better performance for the concept, both in absolute
cooling and in overall cooling efficiency. Atlanta and Baltimore clearly indicate the concept to
produce more cooling during evening hours than could be effectively utilized. For these locations,
this would suggest both interior thermal storage and nighttime dehumidification to further offset
daytime cooling needs.
The very hot climate of Phoenix, however, shows that like Tampa, the concept would only be able
to offset 20 - 30% of daily cooling needs, although seemingly with the potential to essentially
eliminate air conditioning loads during the swing months of April - May and October. Although
Phoenix has less cloud cover, and greater diurnal temperature swing, the ambient evening
temperatures tend to be hotter. Consequently, in this location, the NightCool system often does not
start operation until after midnight. Even so, the concept showed efficient operation in all climates
along with substantial ability to offset cooling needs in more temperate locations.
However, as previously identified in the parametric analysis, the simulation is very sensitive to a
number of critical assumptions which argues for an empirical assessment using test buildings under
realistic weather operating conditions.
34
Evaluation in Small Scale Test Buildings
To verify the potential of the concept,
the radiative cooling system will be
tested in two 10 x 16' test structures.
These highly instrumented buildings
will be located just south of the
Building Science Lab (Figure 20) at
the Florida Solar Energy Center
(FSEC).
One of the test sheds will be the
control structure with a standard attic
with R-19 ceiling insulation and an
asphalt shingle roof with 1:300
ventilation. The experimental unit
will have a white metal roof on metal
battens and a sealed attic, which can
20. The two 10 x 16' small experimental night cool buildings
be convectively linked to the main Figure
are being constructed 20 ft. south of the Building Science Lab.
zone by a powered circulation fan.
Both units will have slab floors, frame walls and small double glazed windows. With some of the
experiments additional mass will be located in the experimental unit to examine how this change
influences performance.
A day/night monitoring protocol would be established with detailed instrumentation listed in Table
12. This would involve measuring air mass flow with leaving and entering temperatures to the sealed
attic space under the radiatively coupled roof. Weather parameters including a pyroheliometer would
be used to determine potential night cooling along with nighttime heat dissipated to the integral
night sky radiator system. Small room air conditioners would be used to supply supplemental
cooling. Internal loads would be simulated by switching on and off interior lamps. Experiments
would be conducted to use dehumidifiers to over-dry the space during periods with excess available
cooling capacity. The overall test schematic is detailed in Figure 21.
35
Figure 21. NightCool test schematic.
1.
2.
3.
5.
6.
White metal roof on metal battens (no decking). Both sides are
surfaced for high emissivity. A temperature probe measures roof
underside temperature.
Small capacity dehumidifer (such as Whirlpool AD40DBK); operates
only during evening hours when thermostat and roof temperature
monitor calls for cooling and attic relative humidity is greater than
50-55%.
Baffled inlet frill from attic for nighttime operation.
Thermostat (compares roof surface temperature and setting to
determine vapor compression vs. nighttime cooling operation).
Room air conditioner operated during daytime.
9.
10.
11.
12.
13.
14.
16.
17.
18.
Interior room air return to attic during evening hours when
NightCool is activated.
Roof-line drip collection system with drain.
Variable speed, dampered fan operated during NightCool to
draw house air into attic.
Attic air connects to cool roof for nocturnal cooling.
R-20 ceiling insulation.
Sealed attic construction with top plate baffles (tested and sealed
system).
Furniture for internal thermal mass.
Concrete slab floor (thermal mass).
Super insulated frame walls to improve load matching with
standard home.
Once data is available on the achievable night sky cooling within the smaller test structures. Specific
applications and concept improvements can be better evaluated in full scale building testing.
36
Table 12
Instrumentation Channel Map for NightCool Experiment
Weather
Dry Bulb
Relative Humidity
Insolation
Wind Speed (roof top)
Wind Direction
Infrared Irradiance
Rain
Ground temperature at 1 ft depth
Roof condensate measurement (south)
Units
EF
%
W/m2
mph
0-360
W/m2
inches
EF
lb.
Thermal
Roof surface temperature
Roof underside temperature (metal roof in experiment, sheathing in control)
Attic air temperature (mid attic)
Ceiling sheet rock temperature (inside surface)
Inlet air temperature to circulation fan
Attic outlet temperature to room
Interior Temperature by control thermostat
Slab interior surface temperature by wall
Slab interior surface temperature at mid width (center)
Slab interior surface temperature at quarter width
EF
EF
EF
EF
EF
EF
EF
EF
EF
EF
Humidity
Attic relative humidity (mid attic)
Interior Humidity by thermostat
%
%
Power (1 Wh/pulse)
AC unit power
Dehumidifier power
Interior attic Circulation fan power (exp only)
Lighting power & indoor circulation fan (Internal loads
Wh
Wh
Wh
Wh
Conclusions
This report has described the configuration and theoretical potential of a novel residential night
cooling concept. Called NightCool, the concept involves using a home's metal roof as a large
radiator to the nightsky to provide effective cooling. Unlike previous more complex night cooling
configurations, the innovation is to selectively link or de-couple the homes internal air node to the
sealed attic under the radiator depending on the prevailing internal and external temperature
conditions. It should also be possible to use the concept for daytime space heating in colder climates
when the attic space is warmer than the interior and space heat would be useful.
37
The report provides a description of the key physical phenomenon involved as well as a numerical
model predicting the systems performance. Under typical nighttime conditions in the Central Florida
region, the concept is shown to produce a cooling rate averaging 2,160 Watts (7,360 Btu/hr) at a
house 78EF interior temperature. The model is exercised given the various physical parameters
governing its performance to determine sensitivity to the various factors. Key weather parameters
are: outdoor temperature, cloudiness and windspeed. It is shown that the key system operation
related parameters influences performance are: system air entering temperature and resulting
radiator temperature and system air flow rate.
An analysis is also performed on hourly TMY2 data for Tampa, Florida which allowed realistic
estimate of system long term performance. The evaluation found that even in Florida's
disadvantageous climate that an average daily cooling of 15 kWh would be available from June
through September with operating COPs of 10.9 indicative of a system SEER of 37 Btu/Wh.
Simulations were also performed in differing climates.
Predicted summertime cooling in Atlanta (50 kWh), Baltimore (62 kWh), and Phoenix (23 kWh)
was superior to that in humid Florida. Seasonal efficiencies in these locations corresponded to
SEERs of 61-89 Btu/Wh.
Finally, we describe a side-by-side experiment using two highly instrumented scale test sheds to
verify the concept. Data from these FSEC facilities should be available by 2006.
38
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